We were on the Summit to Alaska for a 13-night cruisetour starting this past July 3...
We were in Sky Suites on the ship and never had a problem with light or sleeping...From Seward on down (and the Infinity doesn't even take you as far north as Seward), it was dark at Midnight...sure it is a short "night", but nowhere near the 24-hours of daylight you get further north...And the curtaibns pretty much blocked out everything...
Now, for the really weird experience, you need to do the trip we did...We landed in Fairbanks on July 3 and spent two nights there...It NEVER got dark...the sun "officially" set at around 12:45 am and rose about a half an hour later--not going far enough below the horizon to make any real difference...
When we got down to Talkeetna, we had an excellent view of Mt. McKinley from our hotel...Our whole bus was outside at about 12:30 taking photos of the sunset over Mt. McKinley...and you would have thought it was late afternoon...I said, wait about 45 minutes and we can get some pictures of sunrise...I went back to my rom but we left the drapes open to enjoy the views...It NEVER got dark...But, we were tired, we slept anyway, falling asleep at around 2 as the sun was again rising in the sky...
But, by the time we got down to the panhandle (look at a map, it's pretty far south), there was again a distinct day and night...